chapter7

Lectures Overview

  • Lectures include animations and PowerPoint effects that require Slide Show mode.

Chapter 7: Starlight and Atoms

Purpose of Chapter

  • Presents astronomy as an organized understanding.

  • Focuses on how the interaction of light with matter provides clues about celestial objects.

  • Marks a transition to modern astrophysics, applying physics in the study of the sky.

Outline of Chapter

I. Starlight

  • A. Temperature and Heat

  • B. The Origin of Starlight

  • C. Two Radiation LawsII. Atoms

  • A. A Model Atom

  • B. Different Kinds of Atoms

  • C. Electron ShellsIII. Interaction of Light and Matter

  • A. The Excitation of Atoms

  • B. The Formation of a SpectrumIV. Stellar Spectra

  • A. The Balmer Thermometer

  • B. Spectral Classification

  • C. The Composition of the Stars

  • D. The Doppler Effect

  • E. Doppler Shift Mechanism

  • F. Calculating Doppler Velocity

  • G. Shapes of Spectral Lines

Power of Starlight

  • Analyzing light from stars provides insights on:

    1. Total energy output

    2. Surface temperature

    3. Radius

    4. Chemical composition

    5. Velocity relative to Earth

    6. Rotation period

Color and Temperature

  • Stars exhibit different colors (blue, yellow, red) indicating temperature variations (e.g., Rigel, Sun, Betelgeuse).

Black Body Radiation

  • Star light spectrum is approximately thermal, termed black body spectrum.

Laws of Black Body Radiation

  1. Energy emitted increases with temperature: F = s*T^4

  2. Peak wavelength shifts to shorter wavelengths as temperature increases (Wien’s Law).

The Color Index

  • Color index is defined as B – V, comparing brightness in blue (B) and visual (V) bands.

  • Smaller color index indicates hotter stars.

Light and Matter Interaction

  • Spectra of stars show characteristic absorption lines, necessitating understanding atomic structure.

Atomic Structure

  • Consists of a nucleus (protons and neutrons) and electrons. Nucleus contains the mass; the electron cloud occupies space.

Types of Atoms

  • Determined by the number of protons.

  • Most abundant elements: Hydrogen (1 proton), Helium (2 protons).

Electron Orbits

  • Electrons occupy specific energy levels with unique energies for each element.

Atomic Transitions

  • Electrons can transition to higher orbits by absorbing photons of precise energy.

Kirchhoff’s Laws of Radiation

  1. Dense objects emit a continuous spectrum.

  2. Low-density gas emits light at specific wavelengths (emission spectrum).

  3. Continuous spectrum light through cool gas results in absorption spectrum.

Stellar Spectra

  • Stars produce absorption spectra due to cooler surface layers absorbing specific frequencies.

Importance of Specific Absorption Lines

  • Each element presents a unique set of lines to analyze cosmic compositions.

The Balmer Lines

  • Specific hydrogen lines in the visible spectrum indicate transitions between energy levels.

Measuring Stellar Temperatures

  • The strength of absorption lines can determine the temperature of stars.

Spectral Classification

  • Different stars exhibit various absorption line patterns used for classification.

  • Mnemonics assist in remembering star types.

The Doppler Effect

  • Light from moving sources can exhibit blue (toward) or red (away) shifts, providing velocity information.

  • Example: Balmer Alpha line shows shifting based on Earth’s motion.

Doppler Broadening

  • Light absorption lines broaden due to random thermal motion of atoms, primarily influenced by Doppler effect.